ROSE CANKER AND ITS CONTROL. 27 



strength. The areas of bast fibers do not form a complete cylinder, but 

 the inner cortex tissue runs down between them. Just under each bast 

 area there is a region of tissue called phloem. It contains long tubes 

 (sieve tubes) through which the elaborated plant food passes down through 

 the stem from the leaves. Each sieve tube is accompanied by a line of 

 small slender cells (companion cells), which appear in transection as 

 though they were cut out of the corners of the sieve tubes. The remain- 

 ing cells of the phloem are box-like cells called phloem parenchyma. The 

 phloem is bounded below by the cylinder of thin flat cells, the cambium, 

 which marks the line of cleavage between the bark and wood. 



The wood, or xylem, is composed mostly of four kinds of cells: (1) 

 Box-like parenchyma cells which compose the broad medullary rays as 

 well as the narrow rays one cell in width. (2) Long tubes of large diam- 

 eter (tracheae) through which the water mainly passes from the roots 

 to the parts above. The walls are strengthened bj^ spiral or annular 

 thickenings. (3) Vertically elongated cells (tracheids) of smaller diameter 

 and thicker w^alls, also water carriers. These make up the greater portion 

 of the wood. (4) Wood fibers, somewhat smaller in diameter, with thick 

 walls and long tapering points. They cannot be distinguished from the 

 tracheids in transection. Although the walls of all the xjdem elements 

 are heavj^ they are all marked with pits so that liquids have only a thin 

 membrane through which they must pass to go from one cell to the next. 



The pith (not shown in the figure) is composed of cells of only one 

 kind, large or small, somewhat isodiametric (parenchyma). The walls 

 are very thin. 



Path of the Mycelium. — The germ tube, when it attacks the host, is 

 very slender and easily passes between the guard cells down into the 

 stomatal cavity. It could then readily pass between the loose cells of 

 the chlorenchyma and inner cortex, but it does not choose to progress 

 this way. Only rarely has the mycelium been seen progressing for any 

 considerable distance between the cells, but it immediately passes into 

 the cells by means of holes which it is able to dissolve through the walls. 

 From this time on the mycelium is entirely intracellular except for the 

 short distances through which it sometimes passes from one cell to an- 

 other. It branches profusely, but the host 

 cells do not become filled with mycelium. 

 Rarely are more than one or two strands 

 seen in a single cell, except in very old 

 cankers. It is very slender and delicate 

 at first, but in age becomes brown and 

 takes on the various cell forms previously 

 described for the mycelium. It seems to 

 prefer the starch storage cells of the inner ^"'- lO-- Young mycelium in 



, . , . ,. ■ . the cells of the inner cortex. 



cortex, and m cankers ot medium age is 



always found most abundantly in these cells (Fig. 10). However, the 



other cells are not immune. Mycelium may be found quite abundantly 



